1. Field of the Invention
The present invention relates to a planar light source unit and an image display apparatus using such planar light source unit and, specifically, the invention relates to a planar light source unit including a mechanical fastening method for fastening together a resign frame and a lower case without increasing the number of parts, the method being highly reliable in a wide temperature range, and an image display apparatus using the planar light source unit.
2. Description of the Related Art
A liquid crystal display device, a signboard, a guide light and the like are not a display device of a self-light-emission type that the image display part thereof itself emits the light. In these display devices of a non-self-light-emission type, it is necessary to mount a planar light source unit such as a back light on the counter-light-emission side (the rear surface) of the image display part. Now, description will be given below of a planar light source unit to be installed on the back surface of a liquid crystal panel as a representative example of an image display apparatus.
As the planar light source unit, depending on the position of arrangement of a light source, there are available a side light type light source unit and a direct type light source unit. In the planar light source unit of a side light type (which is also referred to as an edge light type), a light source is arranged on the side surface of a case; and, in the planar light source unit of a direct type, a light source is arranged on the counter-light-emission side of a case in such a manner that it is opposed to the liquid crystal panel. Further, the side light type planar light source unit includes a light source unit which uses a light guide plate in order to guide the light from the light source to an opening formed in the case. In the planar light source unit using a light guide plate, the light, which has been guided from a side light portion including a line light source such as a CCFL (Cold Cathode Fluorescent Lamp) or from a point light source such as a light emitting diode (which is hereinafter referred to as an LED), is reflected within the light guide plate and is diffused according to a diffusion pattern provided within the light guide plate, whereby the light can be taken out from the opening in a planar manner.
According to a general planar light source unit, on two or more portions of the peripheral edge of an upper case (which can also be referred to as an upper frame), there are formed fixing pawls by press working, and these fixing pawls are bent and caulked to the pawl receiving portions of the periphery of a lower case (which can also be referred to as a lower frame), thereby fixing the upper and lower cases to each other (JP-A-11-143383, FIG. 21).
Recently, as the display screen of the liquid crystal display device increases, the frame of the case tends to become further narrower, which makes it difficult to keep the mechanical strength of the liquid crystal display device.
In view of this, according to JP-A-11-143383, the peripheral edge of the upper case of the liquid crystal device is bent and caulked onto the peripheral edge surface of the lower case to thereby provide two or more fixing pawls for fixing the upper and lower cases to each other on the peripheral edge of the upper case. Thus, in a state where the fixing pawls are bent and caulked onto the peripheral edge surface of the lower case, after the light guide plate of the planar light source unit is mounted, the fixing pawls each has a length extending up to a position to allow holding of the upper case through the lower case, thereby being able to hold the mechanical strength of the liquid crystal display device (Patent Reference 1, FIGS. 1 and 3).
However, when holding the following parts of the planar light source unit between the upper and lower cases, that is, a light guide plate, an optical sheet and a resin frame having not only a function to reflect the light emitted from the side surface of the light guide plate again onto the light guide plate but also a screwing and mounting function to connect the planar light source unit to an external case, the mechanical fastening method as disclosed in JP-A-11-143383 is not able to provide sufficient strength and reliability. That is, in order to employ a light source such as an LED capable of generating a relatively large amount of heat and thus to attain a wide temperature range of use, it is necessary that, for radiation of the heat, the lower case is increased in size to cover the entire surface of the counter-light-emission side of the case. In this case, especially, the heat generated from the light source at high temperatures is diffused into the lower case and is further diffused to the resin frame as well, so that the resin frame becomes relatively high in temperature. Here, when a member such as a metal case or a printed circuit board having a thermal expansion coefficient different from the material of the resin frame is fixedly secured to the resin frame firmly, the resin frame is distorted due to the difference between the irrespective thermal expansion coefficients. Such distortion, when it becomes large, has a mechanical influence on the reflecting sheet and light guide plate, which gives rise to variations in the luminance of the planar light source unit at high temperatures.